A metal-oxide-semiconductor field-effect transistor (MOSFET) is a transistor used for amplifying or switching electronic signals. In enhancement mode MOSFETs, a voltage drop across the oxide induces a conducting channel between the source and drain contacts via the field effect. The term “enhancement mode” refers to the increase of conductivity with increase in oxide field that adds carriers to the channel, also referred to as the inversion layer. To turn ON the enhancement-mode MOSFET, a voltage is applied to the gate. In depletion mode MOSFETs, the channel consists of carriers in a surface impurity layer of opposite type to the substrate, and conductivity is decreased by application of a field that depletes carriers from this surface layer.
Depletion-mode MOSFETs are doped so that a channel exists even with zero voltage from gate to source. As such, the depletion-mode MOSFET is turned ON without applying a voltage to the gate. To control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device.
Depletion-mode MOSFETs are commonly used in driver circuits. To turn OFF the depletion-mode MOSFET, a negative gate-to-source voltage must be applied. This is typically done by utilizing voltage control to pull the gate voltage to ground.